Proper Sampling - Can i sample like this without destroying reliability?

The population that i need to extract a sample from is all firms from country A with activities in country B. The firms are classified into two categories: as having a subsidiary in country B, or not having a subsidiary in country B (the activity could then be exports, which does not require a subsidiary). The share of firms with subsidiaries is quite small relative to share of firms without.

I already hold the population of the firms with subsidiaries in country B (because this data was available to me). However data on firms with activities but no subsidiary is not readily available and I have to collect that, and I will probably not get access to all firms.

SO my situation is I have the entire population of firms with subsidiaries, and need to collect data of non-subsidiary firms. Most likely I will end up with a sample consisting of all firms with subsidiaries and some share of the firms with no subsidiary. Even without much experience in doing these kinds of studies, I can't help to think that there is some kind is bias/reliability issue when sampling this way (one group: entire group population, other group: some part of group population).

Is my concern entitled? Or is it fine to do it that way? If not, how can I get around the issue?

Any help is greatly appreciated.
To add a little information to my own question, I have done some web searching on the sampling issue: Assuming that the population of firms without subsidiaries is the largest (without any means to by sure of this), the group of firms with subsidiaries will be the minority group, and most likely be oversampled (a new concept to me) since the whole group is included.

Is this a correct understanding, and if I plan to apply logistic regression to the sample, is this a major issue? If it is, how can I get around it?